Define custom theme colors

Defined the Slack custom theme colors according to the
"Nord Style Guideslines" (1).

References:
  (1) https://github.com/arcticicestudio/nord

GH-1

README.md

@@ -9,6 +9,30 @@ Based on the <a href="https://github.com/arcticicestudio/nord">Nord</a> color pa
 
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+<p align="center"><img src="https://raw.githubusercontent.com/arcticicestudio/nord-slack/develop/assets/scrot-hero.png"/></p>
+
+<p align="center"><img src="https://raw.githubusercontent.com/arcticicestudio/nord-slack/develop/assets/scrot-definitions.png"/></p>
+
+<p align="center"><strong>Copy and paste these values to share your custom theme with others:</strong><br><code>#2E3440,#3B4252,#88C0D0,#2E3440,#3B4252,#D8DEE9,#A3BE8C,#81A1C1</code></p>
+
+## Getting started
+
+You can preview the theme in a simplied form at [slack.wopian.me][preview-wopian] and [slackthemes.net][preview-slackthemes].
+
+### Installation
+
+You can install Nord Slack from the desktop app for your workspace at any time by setting the colors as custom theme.
+
+  1. [Sign in][slack-help-sign-in] to the Slack workspace you’d like to customize.
+  2. Click your workspace name in the top left to open the menu.
+  3. Select **Preferences**.
+  4. Choose **Sidebar** from the left-hand menu.
+  5. Copy the Nord Slack theme color defintions and paste them into the input field found at the bottom of the page.
+
+More information on [how to customize your theme][slack-help-customize] can be found in the official [Slack Help Center][slack-help-center] documentation.
+
+<p align="center"><strong>Menu- and item hovering</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-slack/develop/assets/scrcast-hovering.gif"/></p>
+
 ### Contribution
 
 Please report issues/bugs, feature requests and suggestions for improvements to the [issue tracker](https://github.com/arcticicestudio/nord-slack/issues).